Brain Structure - Brain Theory from a Circuits and Systems Perspective - page 13

soma and move along the axon; back propagations back down the dendrites are not
unusual. The axon hillock serves to connect the soma to an axon in an efficient way,
also minimizing interference from inconveniently located transmembrane protons.
The axon is insulated from surrounding ionic solutions with myelination to facili-
tate a more rapid propagation of pulses along longer axons. Signals propagate down
an axon but do not generally return by the same axon. When two-way
communications are required, a separate neural axon is used for return signals.

Neural Signals

From a neuron's view, inputs are excitatory neurotransmitters directed toward
postsynaptic receptors, and outputs are releases of neurotransmitters from presyn-
aptic boutons at the tips or the terminations of axons. But from a circuits view,
neurons are either activated or resting, and in this sense they are defined to be
collections of binary devices, that is, complex gates for Boolean logic. A neuron is a
biological cell with a membrane that accounts for its activity. When activated it
produces a short burst of positive-going pulses [ 2 , 3 ]. When resting, it holds a
steady negative charge; that is, the interior is about 70 mV direct current relative

to their exterior (direct current refers to a small but steady flow of charge leaking
through the membrane).
A neuron may be quite large and is capable of arbitrary Boolean logic, which
means arbitrary combinations of AND, OR, and NOT functions. The neuron
supports differing styles of logic, such as dendritic logic and enabled (soma)
logic, presented later in the topic.
Neural communications are established via synapses; billions of complex neurons
communicate together to achieve a system of consciousness. Solid-state logic is
nothing like neural logic. Although also binary, solid state logic generally relies on
DC voltage levels, a low voltage for false and a higher voltage for true, and does not
depend on pulses, which makes it radically different from a biological system.
The output of an activated neuron is a low-energy, low-frequency burst, some-
times given the fuzzy term action potential. A neural burst consists of low-voltage
pulses, below a few hundred hertz, charging and discharging membrane capaci-
tance between about

70 and +40 mV; each pulse is roughly 2 ms wide. Neural
signals take place within tens of milliseconds with typically ten pulses per burst,
although this number can and has to vary in order to fit into a reasonably efficient
system.

Conclusions

Even though this topic is concerned mainly about logical structure, this chapter has
reviewed the physical structure of a brain, at least the most obvious and interesting
organs. Knowledge like this is important because it inspires conjectures about how